Device and method for range-resolved determination of scattered light, and an illumination mask

ABSTRACT

An illumination mask ( 10   a ) for a device for the range-resolved determination of scattered light, having one or more scattered-light measuring structures ( 11   a ) which respectively include an inner dark-field zone which defines a minimum scattering range, to an associated image-field mask and a corresponding device is provided. Also provided is an associated operating method and a microlithography projection-exposure system having such a device. The scattered-lighter measuring structure in the illumination mask has a scattered-light marker zone ( 20   a ) in the form of a bright-field zone, which on the one hand borders the inner dark-field zone and on the other hand borders an outer dark-field zone, which defines a maximum scattering range. The device may optionally be designed for the multi-channel measuring of scattered light by using a suitable image-field mask and also for multi-channel wavefront measurement, and the detection part may contain an immersion medium. Applications include, for example, the range-resolved determination of scattered light of projection objectives in microlithography projection-exposure systems.

This is a Continuation of U.S. application Ser. No. 10/960,082, filedOct. 8, 2004, which is a Continuation in-Part of InternationalApplication PCT/EP03/10604, with an international filing date of Sep.24, 2003, which was published under PCT Article 21(2) in English, andwhich claims priority of German Patent Application No. 103 36 299.1filed on Aug. 4, 2003. The entire disclosures of the prior applicationsare hereby incorporated into this application by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an illumination mask for a device for therange-resolved determination of scattered light, having one or morescattered-light measuring structures, which respectively include aninner dark-field zone which defines a minimum scattering range, to acorresponding device which comprises the illumination mask for theprovision of measuring radiation on an entry side of a specimen and adetection part for the range-resolved detection of scattered light on anexit side of the specimen, to an image-field mask which can be used forthis, to an associated operating method and to a microlithographyprojection-exposure system having such a device.

2. Description of the Related Art

Such devices and methods are used, for example, for the purpose ofdrawing from the scattered light of optical components detected in arange-resolved fashion conclusions about their optical properties andoptical quality, for example with regard to surface roughnesses,contaminations and material inhomogeneities. Moreover, a scattered-lightportion thus determined can correctively be considered if a spatiallyresolved knowledge, which is as accurate as possible, of the quantity ofradiation actually supplied by an optical system is desired incorresponding optical applications, such as lithographic exposureapparatuses, for example. In the text which follows, the term light isused for the sake of simplicity to denote electromagnetic radiation ofarbitrary wavelength, in particular radiation in the UV or EUV range.

For the purpose of range-resolved determination of scattered light, in aconventional method a non-transparent object which acts as dark-fieldzone is introduced into the beam path of a directed illuminatingradiation having a larger beam cross section compared to the object,such that a shadow image of the object is produced. If the system to beexamined, presently also termed a test component, is an imaging opticalsystem, the non-transparent object is positioned in the object plane ofthe system. The object is imaged by the imaging system onto an image ofthe object in the image plane which may be enlarged or reduced by thereproduction scale and which is denoted as an “aerial image” below, asis also denoted the pure shadow image in the case of test componentswhich are not focusing imaging systems.

Scattering of the illuminating light by the test component has theeffect that scattered light passes laterally into the dark zone of thesurface of the aerial image. The intensity of this scattered lightnormally decreases with increasing distance from the edge to the middleof the aerial image. Depending on the lateral distance from the edge ofthe aerial image, denoted below as scattering distance or scatteringrange, an intensity distribution of the scattered light thus ariseswhich is determined as a function of distance, that is to sayrange-resolved.

In a conventional mode of procedure, a sensor surface is introduced as acomponent of a detection part into the plane in which the aerial imageis produced or detected. For the sake of simplicity, the term imageplane is also used for this plane whenever the test component is not afocusing imaging system. It is important here that the sensor surface issmaller than the aerial image so that a minimum, lateral scatteringdistance remains between the edge of the sensor surface and the edge ofthe aerial image. In consecutive measuring operations, objects ofdifferent size are then positioned such that the size of the aerialimage, and thus the minimum scattering distance, changes. It is knownfor this purpose to provide objects of variable size at a distance fromone another on an otherwise transparent illumination mask. Chromiumsquares of variable edge length, for example, are used as objects. Thesensor surface correspondingly has a quadratic geometry. In order tovary the minimum scattering distance, the illumination mask is displacedsuch that the objects applied to it are brought one after the other intothe beam path and imaged onto the sensor surface centred on the imageside in the plane of the aerial image.

In one of the conventional measuring methods, the entire intensity ofthe scattered light impinging on the sensor surface over a fixed timeinterval is determined integrally for each object size, for example withthe aid of an electro-optical measuring element. Each object size isassigned a minimum scattering distance in the image plane, the maximumscattering distance being infinity in theory for all objects. Thescattered light distribution can be reconstructed from the intensitydistribution as a function of the minimum scattering distance.

Instead of directly measuring the intensity of the scattered light onthe sensor surface, in the case of an alternative conventional measuringmethod a photo-resist layer is provided in the image plane as sensorsurface, and in different measuring operations a respectively differentregion of the photo-resist layer is irradiated with illuminatingradiation of increasing intensity. Then, that limiting value for theintensity is determined in the case of which the aerial image firstvanishes entirely as a structure in the photo-resist layer. Thislimiting value for the intensity of the illuminating light is usedinstead of the light intensity integrated over the sensor surface inorder to determine the scattered light distribution. These and furtherdetails relating to conventional methods of determining scattered lightare to be found in the relevant literature, see, for example, themagazine article by J. P. Kirk, “Scattered Light in PhotolithographicLenses”, SPIE, Volume 2197 (1994), pages 566-572, and the magazinearticle by Eugene L. Church, “Fractal Surface Finish”, Applied Optics,Volume 27, No. 8 (1988), pages 1518-1526.

In the case of the conventional methods outlined above, effects with along scattering range are normally superimposed on effects of short tomedium scattering ranges. The non-transparent objects on theillumination mask certainly define a minimum scattering range, but themaximum scattering range is limited only by the size of the objectfield, that is to say the illumination mask. Moreover, since the squareshave a relatively large spacing from one another, in order not todisturb one another during the measurements, and are typically arrangedin a nonsymmetrical distribution, the long-range scattering lightintensity is a function of the field point, and so systematic measuringerrors can occur.

The devices and methods currently of interest for determining scatteredlight are chiefly used in the field of medium scattering ranges, whatcorresponds, e.g., to typical object-side scattering ranges in a regionfrom approximately 4 μm to approximately 1000 μm and/or image-sidescattering ranges from approximately 1 μm to approximately 250 μm.

Especially for the characterization of optical systems, and inparticular optical imaging systems, not only the scattered-lightbehavior thereof but also the behavior thereof with respect to imagingdefects are of interest. Various wavefront-measurement devices andmethods have been proposed for this, which can be used to find theeffect of the specimen on the wavefront behavior, for example by aninterferometric or Moiré fringe technique, from which the imaging defectbehavior can be deduced. An important field of application is thewavefront measurement of optical systems for microlithography, inparticular projection objectives for microlithographyprojection-exposure systems for the patterning of semiconductor wafers.

Imaging defects of such high-resolution imaging objectives can bedetermined with the required accuracy by using the wavefront-measurementtechnique, in which case radiation of the same wavelength as in theuseful imaging operation of the objective is preferably used for themeasurement, and it has recently become more common to use UV radiationin the wavelength range shorter than about 200 nm. Interferometricmeasurement devices which work in this way with measuring radiation atthe operating wavelength of the specimen may also be referred to asoperational interferometers (OIF) for this reason and they may, forexample, be integrated into the projection-exposure system in question.Various types of interferometric wavefront-measurement devices arecurrently used for this purpose, such as those which are based on shearinterferometry or point diffraction interferometry, or those of theRonchi type, Twyman-Green type or Shack-Hartmann type.

As an alternative to single-channel measurement devices, it is alsopossible to use multi-channel measurement devices with whichmeasurements can be taken from a plurality of field points in parallel,i.e. simultaneously, so that shorter measuring times for full-fieldmeasurements can be achieved compared with single-channel devices. Forexample, the laid-open patent specification WO 01/63233 A2(corresponding to US 2002/0001088) discloses a multi-channel measurementdevice which works with lateral shear interferometry. Such a devicetypically includes an illumination mask, which is to be arranged on theobject side of the specimen and has a structure for generatingwavefronts separately for the various field points, and adiffraction-grating structure to be arranged on the image side of thespecimen for the various field points. With a suitable design of thesystem, the interferograms for the individual field points can be keptsubstantially separate and can be discriminately recorded by adownstream detector, with a detector surface thereof preferably beingplaced at a very short distance from the diffraction-grating structure.

SUMMARY OF THE INVENTION

It is one object of the invention to provide an illumination mask and animage-field mask for a device for the range-resolved determination ofscattered light, as well as such a device and an associated operatingmethod, which permit determination of scattered light with a high rangeresolution and with a good signal-to-noise ratio, and optionallywavefront measurement. It is also an object of the invention to providea microlithography projection-exposure system equipped with such adevice.

These objects are addressed by providing an illumination mask, animage-field mask, a device, a method, and a microlithographyprojection-exposure system as claimed herein.

The illumination mask according to the invention has one or morescattered-light measuring structures in which a scattered-light markerzone in the form of a bright-field zone borders an inner and an outerdark-field zone. By contrast with the above-described conventionalillumination mask, the result of this is to fix not only a minimum, butalso a maximum scattering range. This reduces the long-rangescattered-light component, and this can contribute to a significantimprovement in the signal-to-noise ratio and to the avoidance ofsystematic measuring errors. The geometry of the scattered-light markerzone can be adapted in this case to the geometry of the scattered-lighteffects to be measured or of the test component.

In one embodiment, the illumination mask has a plurality ofscattered-light marker zones with inner and/or outer dark-field zones ofdifferent sizes. It is thereby possible to vary the minimum and/or themaximum scattering range.

In a refinement of the invention, the scattered-light marker zone isdesigned as a ring, something which is advantageous, in particular, whenthe scattered-light effects to be measured have a rotationallysymmetrical structure. This is the case, for example, whenever the testcomponent likewise has a substantially rotationally symmetricalstructure, as is the case with many optical components.

In a development of the invention, the illumination mask has a pluralityof scattered-light marker zones designed as rings and having differentring radii. As a result, the inner and the outer dark-field zones, andthus the minimum and maximum scattering range, vary from ring to ring,and this can be used advantageously in determining the scattered lightin a range-resolved fashion. Since the scattered light generallydecreases exponentially with increasing scattering range, it can befavourable to enlarge the ring width suitably with increasing ringradii, in order thereby to compensate for the decrease in the measuredscattered-light intensity at least partially.

In order to achieve an improvement in the signal-to-noise ratio, thescattered-light marker zones designed as rings can be selected withtheir radii overlapping. Alternatively, the ring radii are selected notto be overlapping, and this increases the range separation whendetermining the scattered light.

The illumination mask may advantageously be designed as a dark-fieldmask with the scattered-light marker zone or zones as bright-field zonesin the mask dark field. It proves to be favourable in this case that inessence only the part of the illumination mask left free from thescattered-light marker zones is of transparent design such that, inparticular, no long-range scattered light is caused by large transparentsurface regions.

In a development of the invention, the scattered-light measuringstructures and one or more dark-field area elements are arranged in theform of a matrix in a regular distribution on the illumination mask suchthat all the nearest neighbours of the dark-field area elements arescattered-light measuring structures. The dark-field area elements canbe used, for example, for calibration purposes. Since all the nearestneighbours of the dark-field area elements are scattered-light measuringstructures, the influence exerted by the surrounding scattered-lightmeasuring structures on the aerial image of a dark-field area elementvirtually does not differ from dark-field area element to dark-fieldarea element.

According to a preferred refinement of the invention, a plurality ofscattered-light measuring structures are arranged distributed over theillumination mask and wavefront-measurement structures, which aresuitable for wavefront measurement of an optical system, are provided inintermediate spaces between the scattered-light measuring structures.

In this way, the illumination mask can be used for combineddetermination of scattered light and wavefront measurement of an opticalsystem. In another configuration, the scattered-light measuringstructures are arranged in matrix form and the wavefront-measurementstructures are arranged in matrix form in intersection areas of thescattered-light measurement-structure matrix.

The image-field mask according to the invention has a plurality ofindividual diaphragm structures, of which at least two diaphragmstructures have diaphragm openings of different size and/or differentshape. The image-field mask therefore provides different diaphragmopenings with which range-resolved measuring of scattered light ispossible for a respective field point in successive measuring processes.For each measuring process, owing to the plurality of availablediaphragm structures which are arranged separately on the image-fieldmask, measurements can advantageously be taken simultaneously at aplurality of field points, and in the extreme case at all of therelevant field points.

In a preferred refinement of the invention, the diaphragm structureshave circular diaphragm openings with two or more different diameters.

In a preferred refinement of the invention, the diaphragm structures arearranged separated on the image-field mask, for example in matrix form,and wavefront-measurement structures for interferometric wavefrontmeasurement of an optical system are provided in intermediate spacesbetween the diaphragm structures. This makes it possible to utilize theimage-field mask both for the determination of scattered light and forthe multi-channel wavefront measurement of an optical system.

The device according to the invention for the range-resolved measuringof scattered light on a specimen has an illumination mask according tothe invention. In particular, the proportion of long-range scatteredlight can therefore be reduced in such a device, which may have anadvantageous effect on the quality of the measurement.

In a development of the invention, the device is designed formeasurement on an optical imaging system, the illumination mask beingarranged in or near the object plane. The detection part has a fieldstop to be arranged in or near the image plane, a micro-objective and asensor surface for scattered-light measurement. The field stop openingis imaged onto the sensor surface by the micro-objective. The field stopscreens off any disturbing, external scattered light which does notoriginate from the scattered-light measuring structure which is imagedonto the image plane by the optical imaging system. The sensor surfacecan be positioned outside the image plane by using the micro-objective.The scattered-light information obtained by the sensor surface can beused by the associated evaluation part to determine in a range-resolvedfashion the scattered light caused by the test component.

In another configuration, the device according to the invention isdesigned for the multi-channel measuring of scattered light, and to thatend, in addition to the illumination mask according to the invention, italso has a corresponding image-field mask, i.e. the diaphragm structuresthereof correspond with scattered-light measuring structures of theillumination mask. This permits parallelized processes of measuringscattered light simultaneously for a plurality of field points.

In a preferred configuration of the invention, the device is alsoconfigured for multi-channel wavefront measurement of the opticalimaging system, i.e. this device can be used to determine both thescattered-light behavior and the imaging defect behavior of the opticalsystem. To that end, the device has an illumination mask according tothe invention with wavefront-measurement illumination-mask structures,and an image-field mask according to the invention withwavefront-measurement image-field mask structures, which correspond withthe wavefront-measurement illumination-mask structures in order to carryout a wavefront measurement. This provides a device which can be used toboth for a multi-channel determination of scattered light and for amulti-channel wavefront measurement of optical systems, in which casefor each measuring process, measurements can advantageously be takensimultaneously at a plurality of field points, and in the extreme caseat all of the relevant field points. This permits a full-fieldmeasurement of optical systems with respect to scattered-light behaviorand imaging defect behavior with relatively short measuring times.

In a preferred refinement of the device according to the invention, thedetection part includes at least one immersion-medium space, for examplein the optical path before and/or after the image-field mask or fielddiaphragm, into which an immersion medium can be introduced.

In a development of the inventive device, the sensor surface isimplemented in a first variant as an electro-optical sensor. In a secondvariant, the sensor surface is designed as a radiation-sensitive layerwhich upon irradiation is changed from a first state to a second state.In the latter case, the detection part has suitable means for measuringthe state of the radiation-sensitive layer. In this second variant, theintensity or dose of the illuminating light can be increased for thepurpose of range-dependent determination of scattered light until theaerial image vanishes as a structure in the radiation-sensitive layer.The corresponding intensity or dose of the illumination and/or thestructural dimensions of the regions exposed by scattered light withdifferent intensities of illumination are then a measure of thescattered light caused by the test component at each respectivescattering distance.

In an operating method according to the invention, the electro-opticalsensor surface is divided into subregions, for example into annularregions. When using a CCD array, this can be achieved by virtue of thefact that a multiplicity of CCD pixels are respectively combined to formcorresponding subregions, and the intensity of the radiation fallingonto the pixels of the respective subregion in a fixed time period isdetermined. The range resolution can be improved by evaluatingsubregions of the sensor surface, since several subregions of differentscattering distance can be evaluated separately on the image side foreach individual scattered-light measuring structure provided on theobject side.

In another operating method according to the invention, the samescattered-light measuring structure is imaged onto different regions ofthe photosensitive layer in various measuring operations with asuccessive increase in intensity of the measuring radiation, and therange-resolved determination of scattered light is performed using statedata for these regions of the photosensitive layer and/or using data forthe intensity or dose of the measuring radiation. Depending on what isneeded, it is possible thereby to make use only of the radiation dosethat is required to cause the image of the scattered-light measuringstructure on the photosensitive layer to vanish, or only of the layerstate data for these regions, or of both parameters in combination forthe range-resolved determination of scattered light.

Another preferred aspect of the invention relates to an operating methodwith which, by employing a device according to the invention configuredappropriately for this, an optical channel system can be studied inmultiple channels with respect to both scattered-light behavior andimaging defect behavior. In this way, both a range-resolveddetermination of scattered light and a wavefront measurement for thedetermination of imaging defects can be carried out on a specimen with arelatively short measuring time.

As an important field of application, the invention relates to amicrolithography projection-exposure system having a device according tothe invention for the range-resolved determination of scattered light ona projection objective of the projection-exposure system, and optionallyfor the wavefront measurement thereof as well. The projection objectivedoes not necessarily need to be removed from the projection-exposuresystem for this. Optionally, with the device then being configuredsuitably for this, the measuring of scattered light and/or the wavefrontmeasurement may be carried out in multiple channels for the full fieldof the objective.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantageous exemplary embodiments of the invention are illustrated inthe drawings and will be described below. In the drawings:

FIG. 1 shows a schematic side view of a device for range-resolvedmeasurement of scattered light at a projection objective of amicro-lithographic projection exposure apparatus, FIG. 2 shows a topview of an illumination mask having a matrix arrangement ofscattered-light measuring structures for a device for range-resolveddetermination of scattered light in accordance with FIG. 1,

FIG. 3 shows a top view of one of the scattered-light measuringstructures of the illumination mask of FIG. 2 having a scattered-lightmarker zone designed as a ring,

FIG. 4 shows schematic top views of a sensor surface of a CCD array fora detection part of the device of FIG. 1 for the purpose of illustratinga subdivision into subregions for measurement purposes,

FIG. 5 shows diagrams for the illustration of a simulation result for arange-resolved determination of scattered light, as can be executedusing the device of FIG. 1,

FIG. 6 a schematic side view corresponding to FIG. 1, but for a variantof the device which is also configured for multi-channel wavefrontmeasurement,

FIG. 7 a plan view of a part of an illumination mask which can be usedin the device of FIG. 6, and

FIG. 8 a plan view of a part of an image-field mask which can be used inthe device of FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a schematic side view of a device for range-resolvedmeasurement of scattered light in operating position at a projectionobjective 1, for example of a micro-lithographic projection exposureapparatus for semiconductor wafer patterning. The device includes anillumination mask 10 positioned in the object plane of the objective 1,a detection part which has a circular field stop 3 positioned in theimage plane of the objective 1, a micro-objective 4 and a CCD array 6 assensor surface, and an evaluation part 7. The micro-objective 4 imagesthe field stop opening onto the CCD array 6 via a deflecting mirror 5.The illuminating light is provided by a conventional illuminating system(not shown).

The illumination mask 10 to be positioned in the object plane of theobjective 1, for example by means of a conventional reticle holder, hasone or more scattered-light measuring structures which in each caseinclude a scattered-light marker zone in the form of an annularbright-field zone between an inner and an outer dark-field zone. In themeasurement mode, a respective one of the scattered-light measuringstructures is positioned in the object plane such that the imaging ofthe scattered-light measuring structure with the aid of the objective 1in the image plane thereof produces an associated aerial image centredin the interior of the circular field stop 3.

Here, the object-side focal plane of the micro-objective 4 correspondsto the field stop plane. In order to avoid beam vignetting at a highnumerical aperture of the objective 1, that is to say for numericalapertures greater than approximately 0.7, the field stop 3 is very thin,that is to say it has a thickness of less than 0.5 μm, preferably ofonly approximately 0.2 μm. On the one hand, the image of the stop edgeis completely visible on the CCD array 6, and so detection is possibleup to a distance of approximately 0.5 μm from the stop edge forimage-side numerical apertures of the objective 1 as far as at least0.95 in a fashion virtually free from vignetting using air as theimage-space medium. Freedom from vignetting is also obtained inapplications in which any appropriate immersion medium, for examplewater, is provided as an image-space medium in the detector part. Inthis case, for immersion systems with a numerical aperture NA>1, themaximum light beam angle achieves similar values as in air with NA<1.The immersion medium, i.e. an optically transparent medium with arefractive index greater than one, may optionally be introduced in thebeam path before and/or after the field diaphragm 3. On the other hand,the image of the diaphragm edge fills out the sensor surface 6 are asmuch as possible, so that as many CCD pixels as possible lie in theimage. Spatial averaging of the scattered-light signal over many CCDpixels helps to improve the signal-to-noise ratio. The numericalaperture of the micro-objective is selected to be greater than theimage-side numerical aperture of the objective 1. If no pupilapodization or other inhomogeneous weighting of beam direction occursover the pupil of the micro-objective 4, the image picked up by the CCDarray 6 of the field stop interior thus constitutes an ideal enlargedimage of the aerial image. The field stop 3 can be used to screen offthe aerial image from disturbing external scattered light. Themicro-objective 4 enlarges the aerial image such that it can beadequately resolved by the CCD array 6.

In a concrete dimensioning example, a pixel spacing of 20 nm withreference to the field stop plane results, that is to say a CCD array 6with 1024×1024 pixels can detect the whole of a circular field stop 3with a diameter of 20 μm, virtually 75% of the CCD pixels being situatedin the image of the stop interior.

The evaluation part 7 serves the purpose of range-resolved determinationof the scattered light caused by the test component with the aid of thescattered-light data picked up by the detection part.

Alternatively, instead of the CCD array 6, it is possible to use anyother desired conventional electro-optical sensor system. As a furtheralternative, it is possible to use as sensor surface aradiation-sensitive layer, in particular a photo-resist layer, whichupon irradiation can be changed from a first state to a second state. Inthe case of such an embodiment, the micro-objective 4 and the field stop3 can be eliminated, and the radiation-sensitive layer is placeddirectly in the image plane during examination at a micro-lithographicprojection objective, for example in the form of a wafer provided withthe photo-resist layer.

FIG. 2 shows a preferred embodiment of the illumination mask 10 for thedevice for the range-resolved determination of scattered light accordingto FIG. 1. In this example, the mask 10 has a plurality ofscattered-light measuring structures 11, which are arranged in matrixform with a regular distribution, most of them having saidscattered-light marker zones 20 while a few of the scattered-lightmeasuring structures are respectively formed by a dark-field surfaceelement 12 which is surrounded only by scattered-light measuringstructures 11 with a scattered-light marker zone 20 as its nearestneighbors. In particular, the matrix arrangement comprises 7×7 fields,which respectively include a scattered-light measuring structure 11 witha scattered-light marker zone 20 or a dark-field surface element 12, theedge fields and every other row and every other column of the matrixarrangement being fully occupied by scattered-light marker zones 20. Thecentres of neighbouring matrix fields have a sufficient spacing a. Theillumination mask 10 is designed as a dark-field object, that is to saythe surface regions between the scattered-light measuring structuresconsist of a material which is not transparent to the illuminatinglight.

As can be seen more clearly from FIG. 3, each scattered-light markerzone 20 is formed as a ring, i.e. it encloses an inner dark-field zone22 and is bounded on the outside by an outer dark-field zone 21. Bothdark-field zones are illustrated by hatching in FIG. 3. Provided in theouter and inner dark-field zones 21 and 22, respectively, for thepurpose of positioning the scattered-light measuring structure 11 arealignment marks 23 which comprise a cruciform bright-field zone which isgenerally negligibly small by comparison with the scattered-light markerzone 20. Four alignment marks 23 are provided abaxially in the exampleshown at a 90° spacing and at the same radial distance from the centre.Alternatively, a central alignment mark can be provided in addition.Omission of the central alignment mark avoids any interference based onlight passing through said mark.

The scattered-light marker zones 20 of the scattered-light measuringstructures 11 on the illumination mask 10 of FIG. 2 have different ringradii, that is to say the inner radius and the outer radius, and thusthe radii of the inner dark-field zone 22 and of the outer dark-fieldzone 21, differ from one scattered-light measuring structure 11 to theother. The ring radii of the various scattered-light marker zones 20 areselected to overlap, for example, that is to say the inner radius of onescattered-light marker zone 20 is larger than the inner radius andsmaller than the outer radius of another scattered-light marker zone,and this contributes to improving the signal-to-noise ratio.Alternatively, the ring radii can also be selected not to overlap, forexample with outer and inner radii which directly adjoin one anothersuch that the range separation is increased during the determination ofscattered light. The selection of scattered-light marker zones 20 whichoverlap or do not overlap in size, is not, of course, limited to the useof rings, but can be used for any desired scattered-light markergeometries.

FIG. 4 shows three schematic top views of the CCD array 6 from FIG. 1,more specifically on the pixel field 30 thereof, for the purpose ofillustrating a selective subdivision of the CCD pixel field 30 intosubregions during measurements of scattered light. The circular image ofthe field stop opening defines an inner, circular pixel field surfaceregion 32 in which the aerial image imaged by the micro-objective issituated. The part 31, remaining outside thereof, of the square pixelfield 30 of the CCD array 6 has no function for the measurement ofscattered light. The inner, circular region 32 is subdivided selectivelyinto subregions for the purpose of measurement and evaluation. In afirst measuring step, only one central circular region 33 of the pixelfield 30 is used for the measurement, as illustrated in the left-handpartial illustration of FIG. 4. In a second measuring step, only onecentralized pixel field annular region 34 is actively switched, asillustrated in the middle partial illustration of FIG. 4. In a thirdmeasuring step, only one second pixel field annular region 35 isactivated, the radius thereof being greater than that of the firstannular region 34, as is shown in the right-hand partial illustration ofFIG. 4. In the same way, further measuring steps continue with aprogressively larger radius of the selectively activated pixel fieldannular region until the entire active region 32 is covered. Inalternative implementations, the consecutive annular regions can beselected with their outer and inner radii overlapping, or notoverlapping.

In a measurement cycle with the aid of the device of FIG. 1, allscattered-light marker zones 20 present on the illumination mask 10 arebrought in sequence into the beam path and imaged by the projectionobjective 1 onto the interior of the field stop 3, and from there ontothe CCD array 6 via the micro-objective 4. All of the circular innersurface region 32 not shadowed by the field diaphragm is divided bymeans of appropriate electronic driving of the CCD array 6 into theseparately activated subregions according to FIG. 4, which arepreferably evaluated simultaneously.

By means of their scattered-light marker zone 20, the scattered-lightmeasuring structures 11 define a minimum and maximum scattering rangewhich differ from one scattered-light measuring structured to another,so that good range resolution can be achieved. A contribution is alsomade to this effect by the decomposition of the CCD array 6 into theselectively evaluable subregions of the surface region 32 relevant tothe measurement. If a number n of scattered-light marking zones 20 arepresent on the illumination mask, and if the surface region 32 of theCCD array 6 relevant to the measurement is decomposed into a number m ofsubregions with a different associated scattering distance in each case,a total number n·m of measured values is obtained which can be used todetermine the range-resolved scattered light. In this process, the lightintensity respectively scattered into a subregion is determined inintegral fashion by averaging over the intensity of the scattered lightof all the pixels in the subregion.

The dark-field area elements 12 are used to calibrate the measuredvalues, such elements being introduced successively into the beam path,just like the scattered-light measuring structures 11, and so thecontribution of the scattered light produced by the dark-field areaelements can be measured for the individual subregions of the CCD arrayand for each pixel thereof. This contribution is subtracted from thecontribution which is produced by the respective scattered-light markerzone 20 for the same subregion or the same pixel as, in the case ofillumination of the test component with the respective scattered-lightmarker zone 20, it is not caused by the scattered light which is due tothe test component and is actually to be measured. The differentialcontribution of scattered light is subjected to time normalization bydividing by a prescribable integration exposure time, and divided by atime-normalized bright-field differential contribution in order togenerate calibrated scattered-light measured values. The saiddifferential contribution is determined as the difference between abright-field measurement for the respective CCD subregion and adark-field measurement with the illumination system switched off, thisdifference being subjected, in turn, to time normalization by dividingby the selected time of exposure or measurement.

As an alternative to the above-described use of an electro-opticalsensor surface, such as the CCD array 6, the invention also comprisesthe use of photosensitive layers as sensor surface, the layer state ofwhich changes detectably upon exposure to the scattered light to bedetected. A photo-resist layer applied to a wafer or another carriermaterial can, for example, serve as photosensitive layer. The spatialresolution of typical photo-resist layers is so high that the aerialimage can be transferred directly to the photo-resist layer even withoutenlargement, that is to say the photo-resist layer is preferablyintroduced into the plane of the aerial image produced. In the case ofuse in the device from FIG. 1, this means that the photo-resist layer ispositioned in the image plane of the objective 1 being examined. Thefield stop 3, the micro-objective 4 and the deflecting mirror 5 can beomitted.

For the purpose of detecting scattered light by means of a photo-resistlayer or another suitable photosensitive layer, the respectivescattered-light measuring structure 11 of the illumination mask 10 isimaged successively onto different regions of the photo-resist layerwith different illumination intensities or illumination doses. For thispurpose, the photo-resist layer is displaced suitably laterally frommeasurement to measurement in a way known per se by a distance which issubstantially greater than the dimension, on the side of the aerialimage, of the relevant scattered-light measuring structure. Thedisplacement is effected, for example, by means of a conventional waferholder in which the wafer with the photo-resist layer applied thereto isfixed. For each scattered-light measuring structure, a measurementseries is undertaken in which the illumination intensity or illuminationdose is increased until the photo-resist layer structure correspondingto the inner dark-field zone 22 has been completely exposed by theinfluence of scattered light, that is to say has reached the same layerstate as the surrounding photo-resist layer annular region, whichcorresponds to the associated scattered-light marker ring 20. Dependingon the type of photo-resist layer, this means, for example, that thephoto-resist layer region corresponding to the inner dark-field zone 21is firstly retained virtually completely in the case of a very lowexposure dosage, whereas the surrounding photo-resist layer ring,corresponding to the scattered-light marker ring 20, is developed away,and with a higher exposure dosage the radius of the retained, innerphoto-resist layer circle becomes ever smaller until this photo-resistlayer circle has entirely vanished at a certain limiting value of theexposure dosage. The radii of the inner photo-resist layer circlesretained in a respective measurement series can subsequently bedetermined in a conventional way, for example in a scanning electronmicroscope.

In a simple variant of evaluation, for each scattered-light measuringstructure use is made as evaluation variable for the range-resolveddetermination of scattered light only of the exposure dosage limitingvalue determined in this way at which the photo-resist layer structurecorresponding to the inner dark-field zone 21 has firstly entirelyvanished. It is sufficient in this case to observe the exposure dosageat which the relevant photo-resist layer structure has completelyvanished. In alternative evaluation techniques, the radii of theretained, inner photo-resist layer circles are determined quantitativelyas a function of the illumination intensity or illumination dosage, andtaken into account additionally or as an alternative to the limitingvalue of the illumination dosage during the evaluation. The quality ofthe evaluation and, in particular, the range separation can thereby besubstantially improved. Taking into account the measured relationship ofthe radii of the remaining photo-resist layer structures as a functionof the illumination dosage corresponds to the above-described mode ofprocedure of dividing a pixel field up into a plurality of separatelyevaluated subregions in the case of the CCD array 6.

The determination of the scattered light distribution function fromwhich it is then possible to deduce the optical quality or the opticalproperties of the test component and, in particular, the scatteringbehaviour thereof is performed subsequently in the usual way by solvingthe appropriate inverse scattering problem. The starting point for thispurpose is an approach which describes the calibrated scattered lightmeasured values as the sum of two components, specifically a first,residual long-range component and a second component which includes thescattered-light distribution to be determined. This second component canbe described by a double integral of the scattered-light distributionover the relevant object-side and image-side surface regions, in thecase of rotationally symmetrical systems, for example, by a first radialintegration over the surface of the scattered-light marker zone 20 beingconsidered, and a second radial integration over the surface of theannular or circular subregion being considered on the CCD array, that isto say the integrand is the range-dependent scattered-light distributionfunction which depends on the lateral differential vector between thescattered position and detector position. The contribution of the secondcomponent is, furthermore, divided by the area of the consideredsubregion of the CCD array, for the purpose of normalization.

In order to solve the inverse scattering problem, an advantageous modeof procedure in the case of rotational symmetry is to use a radiallysymmetrical step function of variable step height as an approach for thescattered-light distribution function. The unknown step heights, andthus the solution vector of the inverse scattering problem, can then bedetermined using customary methods for solving inverse scatteringproblems by means of a least square fit from the calibrated measuredvalues provided with suitable weightings.

FIG. 5 illustrates in the form of a diagram the result of a practicalsimulation which proceeds from the device in accordance with FIGS. 1 to4 and the algorithm, discussed above, for solving the inverse scatteringproblem, illuminating radiation with a wavelength of 193 nm having beenassumed. The left-hand partial illustration shows an assumeddistribution of the relative scattered-light component as a function ofthe scattering distance. The middle partial illustration of FIG. 5 showsthe associated scattering data, calculated by integration, as acharacteristic diagram for the respective detector-side ring region jand illumination-side ring region i. Assuming a realisticsignal-to-noise ratio, these simulated measured values were used toback-calculate the scattered-light distribution by solving the inversescattering problem using the approach explained above. The associatedresult, shown in the right-hand partial illustration of FIG. 5, makes itclear that the original scattered-light distribution is reproduced verywell, and the approach used is therefore realistic and justified.

While the measuring of scattered light is carried out in theaforementioned way for a respective field point in the case of thedevice of FIG. 1, and determination of scattered light is accordinglycarried out at a plurality of field points by measurements taken inchronological succession, the device of FIG. 6 permits multi-channelparallelized determination of scattered light simultaneously for all ofthe relevant field points. Another difference from the device of FIG. 1is that the device of FIG. 6 is also designed for wavefront measurementof the specimen, for example here again the projection objective 1 of amicrolithography projection-exposure system, by multi-channel lateralshear interferometry according to, for example, the aforementionedconventional principle of the multi-channel operational interferometer(OIF). In other words, highly parallelized full-field measuring both ofthe scattered-light behavior and of the imaging defect behavior of theobjective 1 can be carried out by the device of FIG. 6.

The structure of the device of FIG. 6 corresponds to that of FIG. 1 withthe following modifications. Positioned on the object side is anillumination mask 10 a which is provided over the entire field with anillumination-mask structure, as is shown in the partial plan view ofFIG. 7 and will be explained in detail below. On the image side, thedevice of FIG. 6 contains a detector part which, instead of the fielddiaphragm 3 of the device of FIG. 1, comprises an image-field mask 3 aon a quartz support 40. A CCD array 6 a is arranged directly behind thequartz support 40 of the image-field mask 3 a, without interposition ofimaging optics. The image-field mask 3 a has the image-field maskstructure as shown in the partial plan view of FIG. 8, which will beexplained in be detail below.

As can be seen in FIG. 7, the illumination mask 10 a is composed of atwo-dimensional field of illumination-mask unit cells, of which threeunit cells 41 a, 41 b, 41 c lying successively in a row direction aredepicted in the partial view of FIG. 7. Other such unit cells,symbolically indicated in FIG. 7 by the ellipsis, are arrangedsuccessively in a column direction perpendicular to the row direction.Each unit cell consists in turn of a 5×5 matrix of squarescattered-light measuring surface elements arranged with a certainspacing, which respectively function as a scattered-light measuringstructure 11 a with a scattered-light marker zone 20 a or a dark-fieldsurface element 12 a. There are four of these dark-field surfaceelements 12 a in the example which is shown, one of which isrespectively arranged in every other row and every other column of theunit-cell matrix, so that it is surrounded only by scattered-lightmarker zones 20 a as its nearest neighbors. To this extent theillumination mask of FIG. 7 is similar to that of FIG. 2, it beingformed similarly as a dark-field object and with the scattered-lightmarker zones 20 a respectively forming annular bright-field zones. Thatwhich was said above about the scattered-light marker zones 20 of FIGS.2 and 3, to which reference may be made, accordingly applies in respectof the properties and alternatives of the annular scattered-light markerzones 20 a, apart from the fact that they do not have alignment marks.In particular, the annular scattered-light marker zones 20 a havedifferent ring radii, i.e. different radii of the associated innerdark-field zone and the associated outer dark-field zone. The ring radiiof the twenty-one different scattered-light marker zones 20 a of eachunit cell 41 a, 41 b, 41 c are optionally selected to overlap or notoverlap. In the latter case, it is possible to increase the rangeseparation for the determination of scattered light.

In addition to this matrix of surface elements 11 a for thedetermination of scattered light, the illumination-mask unit cells 41 a,41 b, 41 c of FIG. 7 contain a matrix of wavefront-measurement surfaceelements 42, which are arranged in the intersection areas of theintermediate spaces between the mutually separated scattered-lightmeasuring surface elements 11 a, 12 a and which respectively contain anillumination-mask structure 42 suitable for a wavefront measurement. Inthe example which is shown, these wavefront-measurementillumination-mask structures 42 are arranged as five transparent surfacesquares arranged in a checkerboard pattern around a so-called coherencemask structure. As is known, such a coherence mask structure is suitableas a wavefront-generating structure for a wavefront measurement bylateral shear interferometry, a multi-channel full-field measurement ofthe specimen 1 can be carried out by using the arrangement of thesecoherence mask structures 42 in matrix form over the entire illuminationmask. In other words, the illumination mask of FIG. 7 may be used tocarry out both a range-resolved determination of scattered light andmeasuring of multi-channel lateral shear interferometry on the specimen1 by the multi-channel full-field measurement technique.

Conventional types of diaphragms which are used in a conventionalupstream illumination system (not shown) expediently insure that onlythe scattered-light measuring surface elements 11 a of the illuminationmask 10 a are exposed in the scattered-light measuring mode, while thecoherence mask structures 42 remain shadowed, and conversely insure thatonly the coherence mask structures 42 are illuminated inwavefront-measurement operation and the scattered-light measuringsurface elements 11 a remain shadowed. This avoids scattered-lighteffects on the structures intended for one type of measuring operationin the other type of measuring operation.

The image-field mask of FIG. 8 is correspondingly suitable for measuringmulti-channel shear interferometry. Correspondingly to theillumination-mask unit cells 41 a, 41 b, 41 c, the image-field mask ofFIG. 8 is formed by a 1×5 field of unit cells 43 a, 43 b, 43 c, whichrespectively contain five scattered-light measuring surface elements 44arranged mutually separated in a row direction and fivewavefront-measurement surface elements 45. In particular, three suchunit cells 43 a, 43 b, 43 c are arranged successively in the rowdirection in FIG. 8 to form an image-field mask row, and apredeterminable number of such image-field mask rows follow one anotherin a column direction, as indicated by the ellipsis in FIG. 8. In thisway, the wavefront-measurement surface elements 45 are respectivelyarranged in the intersection area between the scattered-light measuringsurface elements 44.

The scattered-light measuring surface elements 44 of the image-fieldmask 3 a respectively form a diaphragm opening structure with atransparent, circular diaphragm opening, while the remaining part ofthese square scattered-light measuring surface elements 44 is nottransparent. In particular, diaphragm openings with different openingdiameters are selected for the five successive scattered-light measuringsurface elements 44 in the row direction of each image-field mask unitcell 43 a, 43 b, 43 c. In the example which is shown in FIG. 8, theopening diameter then decreases for the five diaphragm openingstructures 44 sequenced from left to right.

The wavefront-measurement surface elements 45 of the image-field mask 3a respectively form a diffraction-grating structure. The illuminationmask 3 a therefore contains, interleaved over the entire field of view,a matrix of diaphragm structures 44 for the range-resolved measuring ofscattered light and a matrix of diffraction structures 45 for thewavefront measurement by lateral shear interferometry. Any diffractionstructures known for this application may be used for the diffractionstructure 45, a checkerboard grating structure being selected in FIG. 8by way of an example.

As can be seen from the preceding description of the illumination mask10 a and the image-field mask 3 a, the device of FIG. 6 can be operatedselectively in a highly parallelized scattered-light measuring mode andin a highly parallelized wavefront measuring mode. To that end, theillumination mask 10 a and the image-field mask 3 a are respectivelypositioned on the object side, preferably in or close to an object planeof the objective 1, and on the image side, preferably in or close to animage plane of the objective 1, in corresponding positions according tothe plan views of FIGS. 7 and 8, i.e. the diaphragm structures 44respectively lie in the vicinity of the air image of a scattered-lightmeasuring structure 11 a and the diffraction structures 45 lie in theregion of influence of the wavefront radiation coming from an associatedcoherence mask structure 12 a. Determination of scattered light or awavefront measurement can thus simultaneously be carried out selectivelyfor all of the relevant field points.

In the device of FIG. 6, the CCD array 6 a is primary used as a lightcollector for all of the radiation passing through the image-field mask3 a. The spacings of neighboring diffraction structures 45 of theimage-field mask 3 a are selected to be large enough so that theassociated beams of rays do not overlap on the detector surface of theCCD array 6 a, taking into account the opening angle which may berelatively large when a projection objective 1 with a very highnumerical aperture is being studied. The range separation in thescattered-light measuring mode is facilitated in the device of FIG. 6 byusing the diaphragm structures 44 with different diaphragm openingdiameters on the image-field mask 3 a, instead of selecting different,actively configured CCD pixel field ring regions, as in the device ofFIG. 1, see the preceding description of FIG. 4.

As in the device of FIG. 1, it is also possible to use an immersionmedium in the detection part of the device of FIG. 6, if so required. Inparticular, there may be an immersion medium in the optical path beforethe image-field mask 3 a, i.e. between the objective 1 a to be measuredand the image-field mask 3 a, and/or between the quartz support 40 andthe CCD array 6 a. In the latter case, the immersion medium fills thegap necessarily left between the quartz support and the CCD array 6 a inorder to compensate for irregularities. All media conventionally usedfor this may be employed as an immersion medium, for example water.

The alignment of the illumination mask 10 a relative to the image-fieldmask 3 a is carried out in two stages. In a first step, direct alignmentis carried out by displacing the illumination mask 10 a stepwiserelative to the image-field mask 3 a and recording the associatedmeasuring signal from the CCD array 6 a. Whenever air images ofscattered-light marker zones 20 a with a relatively small internaldiameter are encountered in the vicinity of the diaphragm openings 44with a large external diameter, the measuring signal will increasesignificantly as soon as direct radiation passes through the diaphragmopening 44, even if the misalignment is small. In a possible embodiment,the illumination mask 10 a comprises a matrix of 3×3 unit cells 41 a, 41b, 41 c, . . . and therefore respectively a matrix of 15×15scattered-light measuring surface elements 11 a and 15×15wavefront-measurement surface elements 42. An optimum relative lateralposition is then obtained for each unit cell 41 a, 41 b, 41 c, . . . ,that is to say a 3×3 array of optimum lateral positions for all of theunit cells 41 a, 41 b, 41 c. This allows complete elimination both oftranslation defects and of relative scaling and rotation defects. Acoarse adjustment of the focus may be carried out at this stage, forexample by visual focusing which uses a Moiré fringe technique, to whichend conventional types of grating patterns (not shown) are provided inan associated edge region of the illumination mask 10 a and of theimage-field mask 3 a. In a second step, the fine adjustment is carriedout with respect to distortion and astigmatism defects by using theactual wavefront-measurement data.

The measuring process with the device of FIG. 6 will now be explained byway of an example for a matrix of 11×11 field points. The aforementionedillumination mask 10 a, respectively with a 15×15 matrix ofscattered-light measuring surface elements 11 a andwavefront-measurement surface elements 42 is suitable for this. Elevenscattered-light measuring rows 43 and wavefront-measurement rows 46,with fifteen columns each, are then respectively sufficient for theimage-field mask 3 a.

In the scattered-light measuring mode, the measuring signals of the CCDarray 6 a are recorded for a corresponding central 11×11 matrix ofassociated pairs of scattered-light measuring structures 11 a of theillumination mask 10 a, on the one hand, and diaphragm openingstructures 44 of the image-field mask 3 a, on the other hand. In thiscase, neighboring field points firstly use the air images of differentscattered-light measuring surface elements 11 a of the illumination mask10 a and different diaphragm-structure surface elements 44 of theimage-field mask 3 a. In order to illuminate a respective field pointwith each of the total twenty-five different scattered-light measuringsurface elements 11 a, 12 a of an illumination-mask unit cell 41 a, 41b, 41 c, the illumination mask 10 a is moved sequentially through all ofthe twenty-five lateral positions needed for this. The edge overlap ofthe illumination mask 10 a, which has a size of 15×15 surface elements,of respectively two surface regions on each of the four sides is usedfor this. The five different aperture sizes are sequentially applied foreach field point by moving the image-field mask 3 a horizontally throughinteger multiples of the spacing of the diaphragm structures 44. Theedge overlap on the two sides of the image-field mask 3 a lying oppositeeach other in the translational direction is in turn used for this,which again amounts to two columns, since the image-field mask 3 a hasfifteen columns.

A set of 25·5 measuring values, which result from the twenty-fiveillumination-mask settings combined with the five image-field masksettings and are to be taken sequentially, is obtained for each of the11×11 field points by combining the two measuring steps above. Theperformance capacity of the device of FIG. 6, based on the twenty-onedifferent sizes of scattered-light marker zones 20 a and the fivedifferent sizes of diaphragm openings 44, corresponds to the performancecapacity of the device of FIG. 1. In the device of FIG. 6, theevaluation of the measurement signals is carried out with an evaluationpart 7 a by the same evaluation technique as described above withreference to the example of FIG. 1, with the evaluation part 7 there, towhich reference may be made.

Compared with the device of FIG. 1, the device of FIG. 6 has theadvantage of a significantly shorter total measuring time. Under typicalconditions of, for example, about 12 s for an individual measurementincluding the waiting time, the device of FIG. 6 requires only about 25minutes overall for the total 125 measuring processes and then providesthe full-field information for the range-resolved measuring of scatteredlight. Under the same conditions with the non-parallelized measuringtechnique of the device of FIG. 1, a measurement time or several hourswould be needed in order to obtain the same full-field information.

The device of FIG. 6 also makes it possible to obtain the full-fieldinformation for the imaging defect behavior of the specimen 1 by aparallelized wavefront measurement by means of multi-channel lateralshear interferometry. This then characterizes the objective 1completely, apart from any very long-range scattered-light effects. Withthe device of FIG. 6, for example, projection objectives ofmicrolithography projection-exposure systems of the scanner or steppertype can be characterized for all field points, in respect ofscattered-light behavior and imaging defect behavior, with acomparatively short measuring time. The signal strength and thereforethe signal-to-noise ratio, or the necessary integration time of a CCDcamera being used is likewise less for the device of FIG. 6 comparedwith the sequential measuring technique of the device of FIG. 1, i.e.the resulting signal attenuation is reduced. With a comparable CCDsensitivity, this allows a significant reduction of the CCD integrationtime. Since the device of FIG. 6 does not have any imaging optics in thedetector part, it is also free of any perturbing effects of such imagingoptics, for example generation of scattered light. Similar propertiesand advantages as for the device of FIG. 1, as explained above withreference thereto, are moreover obtained with the device of FIG. 6. Itshould be understood that the invention also covers variants of thedevice of FIG. 6 which use another conventional wavefront measurementtechnique, instead of lateral shear interferometry, to which end thewavefront measurement structures on the illumination mask and theimage-field mask are then respectively selected accordingly.

The device according to the invention can be used not only forrange-resolved determination of scattered light at projection objectivesof micro-lithographic projection exposure apparatuses, but also at anyother desired optical systems. Again, it is possible as an alternativeto the closed annular shape shown to use scattered-light marker zones ofanother, in particular also not rotationally symmetrical shape, adaptedto the geometry of the optical systems to be examined or of the symmetryof their scattering behaviour, for example the shapes of an annularsegment.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allsuch changes and modifications as fall within the spirit and scope ofthe invention, as defined by the appended claims, and equivalentsthereof.

1. An illumination mask for a device for range-resolved determination ofscattered-light, comprising: at least one scattered-light measurementstructure, which respectively includes an inner dark-field zone, whichdefines a minimum scattering range, wherein the at least onescattered-light measuring structure has a scattered-light marker zone inthe form of a bright-field zone, which on the one hand borders the innerdark-field zone and on the other hand borders an outer dark-field zone,which defines a maximum scattering range.